There's a good chance that in the past month or so you'll have been woken from your slumber by the unearthly howl of a copulating fox. This 3am banshee wail is the call of a vixen announcing her readiness to mate. For a fox, foreplay is noisy, sex is brief, but the morning after can really drag. The male dismounts but can't leave because his penis will be locked inside the vixen. This "knot" improves the chance of conception and ensures no other male can mate with her. The two foxes can stand there for some time, fused at the rear with heads at either end like a vulpine pushmi-pullyu.

A pair of stuffed foxes stuck in this odd position is one of many eye-catching exhibits at the Natural History Museum's "Sexual Nature" exhibition, a celebration of the variety and inventiveness of sexual activity among birds, mammals, fish, reptiles and insects. The key to success with a subject like this is the tone, especially at an institution such as the Natural History Museum, which maintains a sometimes uneasy balance between scientific establishment and children's playground.

“Sexual Nature" is as informative as it is unembarrassed, an attitude exemplified by the use of Isabella Rossellini's Green Porno short films, in which she merrily re-enacts some of the weirder animal mating habits like a suggestive children's TV presenter. The film of her ­ecstatically exclaiming "Sadomasochism excites me", while dressed as a snail, goes a long way towards justifying the £8 admission fee.

The science is no afterthought. The exhibition shows why animals go through the rigours of sex rather than procreate asexually and explains Darwin's theory of sexual selection: that the male must impress the female if he wishes to pass on his genes. But this is broken up with tales of extraordinary sexual behaviour. And that's why we're here - so let's start with the leopard slug, a hermaphrodite that has sex while hanging from a tree. It unfurls a penis as long as its body and this gloopily entwines with the penis of its mate. Occasionally, the two penises get tangled, so they'll bite one off.

Sex can seem like a game of constant evolutionary one-upmanship between male and female. Ducks have developed intricate vaginal passages to send unwanted phalluses the wrong way, while male bees leave part of their penis behind to act as a plug. The male diving beetle has produced suckers on his feet to help him stick to the female during copulation, so the female has evolved ridges on her back to stop the male from getting a foothold. The male praying mantis frequently gets his head bitten off during sex, while the male damsel fly has a penis shaped like a bucket to scoop out competitors' sperm. It's vicious stuff.

Most of these tales are told through film and captions, but there are also exhibits such as enmeshed antlers from two caribou that died locked in alpha-male combat and the stuffed mass of London Zoo's regal Guy the Gorilla. Guy is used to illustrate a certain kind of male dominance and he opens a section on sexual politics, which looks at childcare, monogamy, promiscuity and homosexuality - the latter having been recorded in around 450 species.

Given the range of fascinating material, it's a shame the exhibition ends with a section that clumsily tries to apply these behavioural patterns to humans - this feels as tacky as it is tacked-on. Instead, go back and wonder at the mannequin bird, whose mating dance is so complicated and energetic he has to recruit an apprentice to help him through the ordeal. Marvel at the rampant promiscuity of the bonobo ape, which uses sex as a greeting, fare­well, ice-breaker and conflict-resolver. Finally, ponder the sad life of the male anglerfish, little more than nostrils and testes, who fixes himself to a much larger female soon after birth and sits there for life like a parasitic sperm-producing organ. Try and make a screwball anthropomorphic animated comedy out of that. l